The invention relates to a process for the production of fibreboards from coarse wood particles and isocyanate as binder, in which the wood particles are heated under excess water vapour pressure and simultaneously comminuted to fibres which are then delivered with expansion to a drier by means of a blowing pipe, and dried in the drier. Downstream of the drier the fibres are then formed into mats in the customary manner and treated further to form the finished fibreboards. The fibreboards according to the present invention are medium density fibreboards in which wood chips or inexpensive types of wood can be used as wood particles so as to make better use of the raw material wood. As far as their mechanical and physical properties are concerned fibreboards can be compared to wood chipboards, although they display uniform density and fineness of structure throughout their thickness. In addition they have a smooth, coherent surface which allows them to be readily, processed and subjected to a large number of finishing techniques.
EP Patent No. 0 092 699 discloses a process of the kind mentioned in the introduction. The wood particles are digested in a digester under excess water vapour pressure and delivered to a refiner in which the wood particles are comminuted to hot, wet fibres. The hot, wet fibres are delivered from the refiner to a separator via a blowing device comprising a blowing pipe. The digester, the refiner and the blowing pipe are operated under pressure, of for example 8 bars. The temperatures of the hot fibres are in the order of 150 to 160.degree. C. At the end of the blowing pipe leading to the separator reduction in pressure, i.e. expansion, takes place since the separator is operated at normal pressure. In the known process the isocyanate is added as binder In the region of the blowing pipe, i.e. in a region in which excess pressure prevails. The isocyanate is applied to the hot, wet fibres prior to expansion. In the blowing pipe the fibres are preferably vortexed and the isocyanate is homogeneously distributed on the hot, wet fibres, so that lump formation is avoided. Size specks in the finished fibreboard are also thereby advantageously avoided. The addition of the isocyanate binder to the hot, wet fibres in the blowing pipe does however cause pre-curing of the isocyanate, which means that encrustations may form on the inner periphery of the blowing pipe, which thus gradually becomes blocked. Continuous operation is thus considerably disrupted and stoppages are necessary to allow the blowing pipe to be cleaned.
From "Tendenzen der MDP-Plattenerzeugung" (Trends in the manufacture of median density fibreboards) on pages 379 to 382 of the journal 'Holz als Roh- und Werkstoff' (Wood as a raw and processing material) 36 (1978) it is known to add the binder at the exit to the drier. The fibres are thus also hot as a result of the drying process when the binder is added. It is applied to the thin, hot fibres with the aid of sizing blenders commonly employed in the chipboard industry. Although the binder is applied to the hot fibres under normal pressure, problems with pre-curing also occur in this process which are counteracted by the use of large volume intermediate bunkers. One of the main problems encountered in this conventional process of sizing fibres with precondensed urea resins is the formation of size specks, since it is apparently not possible, despite the intense fixing process, to apply the binder to the fibres uniformly and in the required finely distributed form. Also, intense mixing and even the use of large volume intermediate bunkers requires extensive periods of time, which are not conducive to the prevention of pre-curing of the binder.
The invention is based on the problem of providing a process for the production of fibreboards sized with isocyanate, of the kind mentioned in the introduction, in which there is neither the danger of size speck formation nor of any significant pre-curing of the isocyanate.